House of Names

[Cygnus]

The whole idea behind a coat of arms is to be recognized on the battlefield while wearing full armour; since family members generally fought together, having them all bear the same arms really wouldn't make a whole lot of sense. So, as Graham pointed out, coats of arms were and are granted to individuals, not families; though the heraldic achievements of various family members are generally similar to show their membership within that family.

When battles were no longer fought by horsemen in full armour, some areas did away with differencing coats of arms for individuals (I believe Milano was one of the first places to do so), making it so all future members of the armigerous families there would bear identical arms.

Scotland's system of heraldry is often held up as a model of how things should be done and still grants armorial bearings to individuals, differenced using a very complicated system to denote the bearer's position in the family. These bearings are also protected by law, and using another's arms is punishable by a fine as well as confiscation of materials marked with the arms in question.

So, unless your family is both of noble stock and hails from one of those few places to abolish individual coats of arms, the coat of arms you find on similar sites for your surname is not actually yours and it may well be illegal for you to use it.

EDIT: For the sake of completeness, I feel I should say that I neglected to mention that the Netherlands (among other nations) was notorious for its unregulated heraldry (particularly among the merchant classes) even before the system of differencing arms was abolished in other locales.